In a biosignal measurement apparatus that measures a potential signal (hereinafter, referred to as biosignal) such as an electroencephalogram, an electromyogram, an electrocardiogram, and a body fat percentage by bringing a living body (animal including a human being) into contact with an electrode, a contact resistance exists between the electrode and a surface of the living body.
The contact resistance affects a biosignal detected by an electrode particularly in the case where a resistance value is large, so a surface of a living body and the contact surface of an electrode are generally coated with a conductive paste, for example, thereby reducing the contact resistance. However, it is difficult to completely eliminate the contact resistance, so the contact resistance is measured before a biosignal is measured.
For example, Non-Patent Document 1 discloses a digital electroencephalograph that applies an alternating-current signal to an electrode from an alternating-current voltage supply through a resistor to obtain a value of impedance (contact resistance) from a potential difference in the contact resistance. That is, in the digital electroencephalograph, the applied voltage is divided by the resistor and the contact resistance.    Non-Patent Document 1: “Secret of digital electroencephalograph” written by Kazuteru Yanagihara, (online), Kanto Study Group for Neurophysiological Tests (retrieved on Aug. 1, 2011)